1. Field of the Invention
This invention relates to a magnetic head usable for magnetic recording devices, such as magnetic disk devices.
2. Description of the Related Art
Recording density in magnetic recording has been increasing at an annual rate of 60% since the beginning of the 1990s due mainly to the commercial availability of magnetoresistive (MR) heads as magnetic heads, and to the adoption of partial response maximum likelihood (PRML) as a signal processing method.
The metal-in-gap head (hereinafter referred to as MIG head for short), on the other hand, which is manufactured by forming metallic magnetic layers with high saturated magnetic density on both sides of a magnetic gap between ferrite cores with a sputtering process, has been also enjoying an increasing demand due to its low cost and ample availability, and as a result of technological improvements, such as increased saturation magnetic flux density of the metallic magnetic layers.
The MIG head has also succeeded in achieving a surface recording density of 600 Mb/in.sup.2 by using in conjunction with PRML. With the MIG heads, metallic magnetic layers formed on both sides of the gap are made of the same material, that is, FeAlSi with a saturation magnetic flux density of about 1.1 T(Tesla). More recently, however, the saturation magnetic flux density of metallic magnetic layers for MIG heads has been substantially improved as the material has been switched from conventional FeAlSi to FeTaN having a saturation magnetic flux density of 1.5 T. As a result, recording magnetic field intensity has been improved, and recording properties for high coercive-force medium have also been improved accordingly, leading to higher density recording.
With the introduction of PRML as a signal processing method, linear recording density has been substantially increased, compared with the conventional peak detection method, bringing about the problem of non-linear transition shift (NLTS for short), in which recorded magnetization transition point is shifted from a predetermined location.
It is generally believed that there are the following five phenomena as to NLTS during magnetic recording:
(1) DC erasing effect
When a magnetic recording medium has been DC-erased prior to writing, whether the direction of magnetization during writing is in the same direction as, or in the opposite direction to, the DC-erasing direction determines the ease or difficulty of magnetization. The flux reversal or magnetization transition in the direction opposite to DC erasing direction tends to be delayed.
(2) One-bit before effect
When there is a flux reversal one-bit before the location where data is written, the written flux reversal is advanced by the influence of the preceding flux reversal.
(3) Two-bit before effect
When there is a flux reversal two-bits before the location where data is written, the written flux reversal is shifted. The advance shift occurs if the flux reversal at a location two-bits before is in the same direction as the flux reversal at the current location, and the delay shift occurs if in the opposite direction to the flux reversal at the current location.
(4) Broadening effect
When there is a flux reversal one-bit before data is written, the gradient of magnetic field is reduced due to the demagnetizing effect, leading to the increased width of magnetization transition shift.
(5) Partial erasure
When there is a flux reversal one-bit before or after the current location, the amplitude is reduced as a result of mutual interference. When there are flux reversals one-bit both before and after the current location, the amplitude is reduced by a factor of two.
NLTS has a great effect on error rate in the PRML circuit (signal processing). The conventional data restoration method by detecting peak locations does not greatly affect error rate so long as the peak shift is within the interval of one bit. Data restoration in the PRML circuit, on the other hand, requires the peaks be accurately positioned at the location they must be. Whereas a linear shift can therefore be corrected by an equivalent circuit, a non-linear transition shift (NLTS) involving a large amount of shift (more than 30%) due to its non-linearity would degrade error rate because it does not permit compensation by an equivalent circuit.
When a recording magnetic field leaked from the magnetic poles of the magnetic head spreads broadly, the magnetization transition region (flux reversal region) is broadened when the data are written into the recording medium. This causes the location of peaks being detected to change, or the peak height (reproduction output) to lower, resulting in a phenomenon close to (4) above and an increase in NLTS. As a result, error rate in the PRML, circuit deteriorates; the closer the intervals of bits and the higher the recording density, the more remarkably does this phenomenon occur. To cope with this, various measures, including changing the shape of the core, have been taken to make the recording field that leaks from the magnetic poles of the magnetic head sharper or steeper.
A magnetic head slider is often used as a magnetic head for recording and reproducing information on recording media. The magnetic head slider normally has two parallel flying rails on a surface facing a recording media. A slit is formed on one of the flying rails in the rear of the slider, and the core tip of the magnetic head is inserted into the slit and fixed with bonding glass.
The core tip comprises an I-shaped core (I core) and a C-shaped core (C core); both being made of an oxide magnetic material, such as ferrite, and opposing with each other with a magnetic gap between them. Metallic magnetic layers made of the same material are formed on the surfaces facing the magnetic gap of the I and C cores. The metallic magnetic layer on the I core is butted against the metallic magnetic layer of the C core, facing each other, and bonded together with bonding glass. On the core tip wound is a wire winding to record and reproduce signals.
Since the I core and the C core are disposed facing each other with a gap between them, the magnetic flux density of the metallic magnetic layer on the I core is higher at the location they face each other than that of the metallic magnetic layer of the C core. As the recording current increases, the metallic magnetic layer on the I core is apt to be saturated well before the metallic magnetic layer on the C core. This causes widened recording magnetic field and recording demagnetization, lowering both reproduction output and overwrite characteristics, and increasing NLTS. This phenomenon appears remarkably with increases in linear recording density as a result of the application of PRML.